The present invention relates to an apparatus and a process for restoring operation of the nozzles of an ink jet print head.
As is known, the nozzles of an ink jet print head are subject to disadvantages which cause a deterioration in and/or interrupt operation thereof. The main causes of malfunctioning are as follows:
(a) blockage of the nozzles due to the ink drying out;
(b) the presence of air bubbles in the nozzles or in the vicinity thereof in the conduit of the printing element; the bubbles alter the hydraulic characteristics of the conduit with a consequential impairment of the level of efficiency of the printing element;
(c) retention of residues of liquid ink within the nozzles on the front surface of the printing elements; such residues interfere with the ink meniscus in the nozzle and cause expulsion of the drops of ink to be irregular.
Various methods for restoring optimum conditions of operation of the ink jet printing elements have been proposed, mainly directed at eliminating air bubbles from the nozzles. European patent application EP-A No. 45832 describes a method which provides for using two separate frequency generators, one for generating the frequency of the pulses for the printing operation and another for generating a higher frequency for eliminating the bubbles. Since that method excludes the use of a suction effect on the ink combined with high-frequency excitation of the head, it is ineffective in some circumstances, especially when the bubbles strongly cling to the walls of the conduits at irregularities in the internal surface of the conduits. Moreover that method uses a highly complicated and expensive circuit.